When collecting quantitative data you can use a statistical formula to give you a rough estimate of the sample size you will need. When collecting qualitative data, however, the rule of thumb is to have as representative of a sample as possible that will allow you to exhaust the knowledge you could learn from the population.
Macroinvertebrate sampling provides a relatively easy way to assess the water quality of a stream. The types and numbers of macroinvertebrates (mostly insect larvae/nymphs) that form the biological community at a particular stream location are shaped by both the nature of the stream and the quality of water that drains into the stream.